1. Field of the Invention
The present invention relates to a new and improved electric fuse for protection of circuits and more particularly to a time delay fuse having improved short circuit performance and reduced operating temperature.
2. Background
Time delay fuses are characterized by permitting an overload in-rush or surge current to flow through the fuse without interrupting the circuit or clearing the fuse. Such fuses, however, will clear in response to relatively moderate constant current overloads.
Time delay fuses are important for protecting circuits for various types of motors, radio and television receivers and other electrical and electronic devices which experience large surge currents when a power source is connected to energize the device. Shortly after connection to a power source, these devices typically reach normal operating conditions and use a relatively steady flow of normal current considerably lower than the surge current. In such a device, it is not desirable for the fuse to clear too quickly when the power source is applied, but rather a time delay should be provided before clearing.
There have been several attempts to design suitable time delay fuses. For example, U.S. Pat. No. 3,869,689 discloses a fuse including an insulated wire closely wrapped around a resistance wire. Melting insulation plays a role in the performance of this fuse. The difficulties in controlling melting of insulation results in a somewhat less predictable fuse operation.
Another time delay fuse is illustrated in U.S. Pat. No. 4,237,440. The fuse disclosed in this patent includes two cores of insulating material with a figure eight configuration. Time delay is obtained by increasing the diameter and the length of the single wire. However, the process of braiding a single wire around a pair of cores is cumbersome, difficult and relatively expensive.
A fuse defined by a wire wrapped on another wire is illustrated in U.S. Pat. No. 3,267,238. The two wires are of dissimilar materials and one wire is wrapped around the other to provide continuous contact between the two wires. The first wire is of high resistance and low coefficient of thermal expansion, and the second is of a low resistance thereby providing a delaying effect. However, the use of these two coated dissimilar wires increases the complexity and cost of the fuse.
In U.S. Pat. No. 4,057,775, a fusible wire is wrapped by a second wire and the resultant wrapped wire is spirally wound over a highly heat conductive rod-like member which acts as a heat sink to provide time delay. Such a device may also be difficult to manufacture thereby increasing the cost.
A time delay fuse with a single wire wrapped around a glass fiber core is illustrated in U.S. Pat. No. 4,177,444. A similar winding of a single wire about a fiber core is illustrated in U.S. Pat. No. 3,845,439. A very thin single silver wire wrapped around a core is illustrated in U.S. Pat. No. 3,858,142 and a similar fuse is illustrated in U.S. Pat. No. 4,189,696. A fuse including a single wire wrapped around a core, but with the spacing of the coils of the wire varied as illustrated in U.S. Pat. No. 4,034,329. A similar fuse but with a cruciform cross section in combination with an indicating fuse is illustrated in U.S. Pat. No. 3,614,699. Fuses including a single coated or bare wire wrapped around a core are also illustrated in U.S. Pat. No. 1,629,266, and British Patent Specification No. 77,125. The basic principal of operation of element designs which incorporate a single wire wrapped onto a core is that the time delay is obtained by increasing the length and diameter of the wire and therefore the mass. However, this tends to adversely affect short circuit performance.
In U.S. Pat. No. 4,517,544, a cylindrical ceramic core with one or more short wires laid parallel to the longitudinal axis of the core is wrapped by a second longer wire which holds the first short wire to the core. A disadvantage of this design is that the first wire, due to its round configuration, tends to become embedded in the core leaving only a small surface exposed for soldering the wire to the ferrule. The fact that the core material tends to repel solder also increases the severity of this problem.